1. Field of the Invention
The present invention relates to a powdery coating glass material and a coating glass paste for use as a coating to protect thick film print circuits used in electronic parts, and a coating glass composition prepared by the coating glass material and glass paste, and in particular to (i) a ZnO-V.sub.2 O.sub.5 -B.sub.2 O.sub.3 -SiO.sub.2 based powdery coating glass material, (ii) a coating glass paste using the powdery coating glass material which is particularly improved in that it is hardly reduced in the course of the firing thereof for the formation of a coating glass composition in a nitrogen atmosphere, and (iii) the coating glass composition prepared by the above coating glass material and glass paste. By the use of the powdery coating glass material, the coating glass paste or the coating glass composition, the use of copper as a material for conductors, which is easily oxidized, but superior in electrical characteristics and reliability to conventionally employed materials, becomes possible.
2. Description of the Prior Art
A thick film print circuit comprising resistors and conductors formed on a ceramic substrate requires protection from external electrical, chemical, and mechanical damage. Glass is commonly used as a coating material to provide such protection.
Conventionally, silver-palladium is mainly used as a conductive material, but in recent years, as technological advances in the manufacture of electronic parts have made rapid progress, the necessity for high densification of the circuitry has arisen. The use of copper has been proposed because its electrical characteristics and reliability are superior to silver-palladium. However, copper is so easily oxidized that the firing of a circuit comprising a copper conductor for the protection thereof with a coating glass composition has to be carried out in an non-oxidizing atmosphere, such as in an atmosphere of nitrogen.
As such coating glass compositions, conventionally a PbO-B.sub.2 O.sub.3 -SiO.sub.2 based glass composition as disclosed in Japanese Patent Publication 63-10104, a ZnO-B.sub.2 O.sub.3 -SiO.sub.2 based glass composition as disclosed in Japanese Laid-Open Patent Application 62-31903, and a CdO-B.sub.2 O.sub.3 -SiO.sub.2 based glass composition have been proposed.
However, when the PbO-B.sub.2 O.sub.3 -SiO.sub.2 based glass composition is fired for the formation of a glass coating on circuits in a nitrogen atmosphere, the lead oxide contained in the composition is easily reduced and lead is separated out so that the characteristics of the glass coating are subject to radical deterioration.
When the ZnO-B.sub.2 O.sub.3 -SiO.sub.2 based glass composition is used as a coating composition, the resistance of the coated resistors varies considerably before and after the coating, that is, the resistance changing rate thereof is significantly high. Because of this high resistance changing rate, there is the problem of difficulty in setting a desired resistance of the resistor.
In the case of the CdO-B.sub.2 O.sub.3 -SiO.sub.2 based glass composition there is concern about adverse effects on the human body from the use of CdO, as well as the fact that this material is not desirable because of its potential for environmental pollution.